Reference timing determination based on sidelink propagation delay

ABSTRACT

In an embodiment, a UE establishes, with a peer sidelink UE, at least one sidelink communications link that each comprises one or more hops. The UE determines estimates a propagation delay between the UE and the peer sidelink UE based in part upon a relationship between a propagation time, between the UE and the peer sidelink UE, and Reference Signal Received Power (RSRP) irrespective of whether the UE remains synchronized with respect to the network clock.

CROSS-REFERENCE TO RELATED APPLICATION

The present Application for Patent is a Continuation of U.S.Non-Provisional application Ser. No. 16/939,760, entitled “REFERENCETIMING DETERMINATION BASED ON SIDELINK PROPAGATION DELAY”, filed Jul.27, 2020, which in claims the benefit of U.S. Provisional ApplicationNo. 62/887,466, entitled “REFERENCE TIMING DETERMINATION BASED ONSIDELINK PROPAGATION DELAY”, filed Aug. 15, 2019, each of which isassigned to the assignee hereof and hereby expressly incorporated byreference herein in its entirety.

TECHNICAL FIELD

Various aspects described herein generally relate to reference timingdetermination based on sidelink propagation delay.

BACKGROUND

Wireless communication systems have developed through variousgenerations, including a first-generation analog wireless phone service(1G), a second-generation (2G) digital wireless phone service (includinginterim 2.5G and 2.75G networks), a third-generation (3G) high speeddata, Internet-capable wireless service and a fourth-generation (4G)service (e.g., Long Term Evolution (LTE) or WiMax). There are presentlymany different types of wireless communication systems in use, includingCellular and Personal Communications Service (PCS) systems. Examples ofknown cellular systems include the cellular Analog Advanced Mobile PhoneSystem (AMPS), and digital cellular systems based on Code DivisionMultiple Access (CDMA), Frequency Division Multiple Access (FDMA), TimeDivision Multiple Access (TDMA), the Global System for Mobile access(GSM) variation of TDMA, etc.

A fifth generation (5G) mobile standard calls for higher data transferspeeds, greater numbers of connections, and better coverage, among otherimprovements. The 5G standard, according to the Next Generation MobileNetworks Alliance, is designed to provide data rates of several tens ofmegabits per second to each of tens of thousands of users, with 1gigabit per second to tens of workers on an office floor. Severalhundreds of thousands of simultaneous connections should be supported inorder to support large sensor deployments. Consequently, the spectralefficiency of 5G mobile communications should be significantly enhancedcompared to the current 4G standard. Furthermore, signaling efficienciesshould be enhanced and latency should be substantially reduced comparedto current standards.

Some wireless communication networks, such as 5G, support operation atvery high and even extremely-high frequency (EHF) bands, such asmillimeter wave (mmW) frequency bands (generally, wavelengths of lmm to10 mm, or 30 to 300GHz). These extremely high frequencies may supportvery high throughput such as up to six gigabits per second (Gbps). Oneof the challenges for wireless communication at very high or extremelyhigh frequencies, however, is that a significant propagation loss mayoccur due to the high frequency. As the frequency increases, thewavelength may decrease, and the propagation loss may increase as well.At mmW frequency bands, the propagation loss may be severe. For example,the propagation loss may be on the order of 22 to 27 dB, relative tothat observed in either the 2.4 GHz, or 5 GHz bands.

SUMMARY

An embodiment is directed to a method of operating a user equipment(UE), comprising establishing, with a peer sidelink UE, at least onesidelink communications link that each comprises one or more hops,estimating, while the UE is synchronized with respect to a networkclock, a propagation delay between the UE and the peer sidelink UE basedin part upon a relationship between a propagation time, between the UEand the peer sidelink UE, and Reference Signal Received Power (RSRP),and determining, while the UE is synchronized with respect to thenetwork clock, a reference timing based on the estimated propagationdelay.

Another embodiment is directed to a method of operating a user equipment(UE), comprising establishing, with a peer sidelink UE, at least onesidelink communications link that each comprises one or more hops,estimating, while the UE is unsynchronized with respect to a networkclock, a propagation delay between the UE and the peer sidelink UE basedin part upon a relationship between a propagation time, between the UEand the peer sidelink UE, and Reference Signal Received Power (RSRP),and determining, while the UE is unsynchronized with respect to thenetwork clock, a reference timing based on the estimated propagationdelay.

Another embodiment is directed to a user equipment (UE), comprisingmeans for establishing, with a peer sidelink UE, at least one sidelinkcommunications link that each comprises one or more hops, means forestimating, while the UE is synchronized with respect to a networkclock, a propagation delay between the UE and the peer sidelink UE basedin part upon a relationship between a propagation time, between the UEand the peer sidelink UE, and Reference Signal Received Power (RSRP),and means for determining, while the UE is synchronized with respect tothe network clock, a reference timing based on the estimated propagationdelay.

Another embodiment is directed to a user equipment (UE), comprisingmeans for establishing, with a peer sidelink UE, at least one sidelinkcommunications link that each comprises one or more hops, means formeans for estimating, while the UE is unsynchronized with respect to anetwork clock, a propagation delay between the UE and the peer sidelinkUE based in part upon a relationship between a propagation time, betweenthe UE and the peer sidelink UE, and Reference Signal Received Power(RSRP), and means for determining, while the UE is unsynchronized withrespect to the network clock, a reference timing based on the estimatedpropagation delay.

Another embodiment is directed to a user equipment (UE), comprising amemory, at least one transceiver, and at least one processor coupled tothe memory and the at least the transceiver, the at least one processorconfigured to establish, with a peer sidelink UE, at least one sidelinkcommunications link that each comprises one or more hops, estimate,while the UE is synchronized with respect to a network clock, apropagation delay between the UE and the peer sidelink UE based in partupon a relationship between a propagation time, between the UE and thepeer sidelink UE, and Reference Signal Received Power (RSRP), anddetermine, while the UE is synchronized with respect to the networkclock, a reference timing based on the estimated propagation delay.

Another embodiment is directed to a user equipment (UE), comprising amemory, at least one transceiver, and at least one processor coupled tothe memory and the at least the transceiver, the at least one processorconfigured to establish, with a peer sidelink UE, at least one sidelinkcommunications link that each comprises one or more hops, estimate,while the UE is unsynchronized with respect to a network clock, apropagation delay between the UE and the peer sidelink UE based in partupon a relationship between a propagation time, between the UE and thepeer sidelink UE, and Reference Signal Received Power (RSRP), anddetermine, while the UE is unsynchronized with respect to the networkclock, a reference timing based on the estimated propagation delay.

Another embodiment is directed to a non-transitory computer-readablemedium containing instructions stored thereon, which, when executed by auser equipment (UE), cause the UE to perform actions, the instructionscomprising at least one instruction configured to cause the UE toestablish, with a peer sidelink UE, at least one sidelink communicationslink that each comprises one or more hops, at least one instructionconfigured to cause the UE to estimate, while the UE is synchronizedwith respect to a network clock, a propagation delay between the UE andthe peer sidelink UE based in part upon a relationship between apropagation time, between the UE and the peer sidelink UE, and ReferenceSignal Received Power (RSRP), and at least one instruction configured tocause the UE to determine, while the UE is synchronized with respect tothe network clock, a reference timing based on the estimated propagationdelay.

Another embodiment is directed to a non-transitory computer-readablemedium containing instructions stored thereon, which, when executed by auser equipment (UE), cause the UE to perform actions, the instructionscomprising at least one instruction configured to cause the UE toestablish, with a peer sidelink UE, at least one sidelink communicationslink that each comprises one or more hops, at least one instructionconfigured to cause the UE to estimate, while the UE is unsynchronizedwith respect to a network clock, a propagation delay between the UE andthe peer sidelink UE based in part upon a relationship between apropagation time, between the UE and the peer sidelink UE, and ReferenceSignal Received Power (RSRP), and at least one instruction configured tocause the UE to determine, while the UE is unsynchronized with respectto the network clock, a reference timing based on the estimatedpropagation delay.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the various aspects described herein andmany attendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanying drawingswhich are presented solely for illustration and not limitation, and inwhich:

FIG. 1 illustrates an exemplary wireless communications system,according to various aspects.

FIGS. 2A and 2B illustrate example wireless network structures,according to various aspects.

FIG. 3A illustrates an exemplary base station and an exemplary userequipment

(UE) in an access network, according to various aspects.

FIG. 3B illustrates an exemplary server according to various aspects.

FIG. 4 illustrates an exemplary wireless communications system accordingto various aspects of the disclosure.

FIG. 5 illustrates a sidelink communications network in accordance withan embodiment of the disclosure.

FIG. 6 illustrates a sidelink communications network in accordance withanother embodiment of the disclosure.

FIG. 7 illustrates successively higher timing errors along hops of thesidelink communications network in accordance with an embodiment of thedisclosure.

FIG. 8 illustrates an example frame structure that supports sidelinksynchronization signals in accordance with an embodiment of thedisclosure.

FIG. 9 illustrates an exemplary process of determining a referencetiming according to an aspect of the disclosure.

FIG. 10 illustrates an exemplary process of determining a referencetiming according to another aspect of the disclosure.

FIG. 11 illustrates an example implementation of the processes inaccordance with an embodiment of the disclosure.

DETAILED DESCRIPTION

Various aspects described herein generally relate to reference timingdetermination based on sidelink propagation delay.

These and other aspects are disclosed in the following description andrelated drawings to show specific examples relating to exemplaryaspects. Alternate aspects will be apparent to those skilled in thepertinent art upon reading this disclosure, and may be constructed andpracticed without departing from the scope or spirit of the disclosure.Additionally, well-known elements will not be described in detail or maybe omitted so as to not obscure the relevant details of the aspectsdisclosed herein.

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any aspect described herein as “exemplary”is not necessarily to be construed as preferred or advantageous overother aspects. Likewise, the term “aspects” does not require that allaspects include the discussed feature, advantage, or mode of operation.

The terminology used herein describes particular aspects only and shouldnot be construed to limit any aspects disclosed herein. As used herein,the singular forms “a,” “an,” and “the” are intended to include theplural forms as well, unless the context clearly indicates otherwise.Those skilled in the art will further understand that the terms“comprises,” “comprising,” “includes,” and/or “including,” as usedherein, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

Further, various aspects may be described in terms of sequences ofactions to be performed by, for example, elements of a computing device.Those skilled in the art will recognize that various actions describedherein can be performed by specific circuits (e.g., an applicationspecific integrated circuit (ASIC)), by program instructions beingexecuted by one or more processors, or by a combination of both.Additionally, these sequences of actions described herein can beconsidered to be embodied entirely within any form of non-transitorycomputer-readable medium having stored thereon a corresponding set ofcomputer instructions that upon execution would cause an associatedprocessor to perform the functionality described herein. Thus, thevarious aspects described herein may be embodied in a number ofdifferent forms, all of which have been contemplated to be within thescope of the claimed subject matter. In addition, for each of theaspects described herein, the corresponding form of any such aspects maybe described herein as, for example, “logic configured to” and/or otherstructural components configured to perform the described action.

As used herein, the terms “user equipment” (or “UE”), “user device,”“user terminal,” “client device,” “communication device,” “wirelessdevice,” “wireless communications device,” “handheld device,” “mobiledevice,” “mobile terminal,” “mobile station,” “handset,” “accessterminal,” “subscriber device,” “subscriber terminal,” “subscriberstation,” “terminal,” and variants thereof may interchangeably refer toany suitable mobile or stationary device that can receive wirelesscommunication and/or navigation signals. These terms are also intendedto include devices which communicate with another device that canreceive wireless communication and/or navigation signals such as byshort-range wireless, infrared, wireline connection, or otherconnection, regardless of whether satellite signal reception, assistancedata reception, and/or position-related processing occurs at the deviceor at the other device. In addition, these terms are intended to includeall devices, including wireless and wireline communication devices, thatcan communicate with a core network via a radio access network (RAN),and through the core network the UEs can be connected with externalnetworks such as the Internet and with other UEs. Of course, othermechanisms of connecting to the core network and/or the Internet arealso possible for the UEs, such as over a wired access network, awireless local area network (WLAN) (e.g., based on IEEE 802.11, etc.)and so on. UEs can be embodied by any of a number of types of devicesincluding but not limited to printed circuit (PC) cards, compact flashdevices, external or internal modems, wireless or wireline phones,smartphones, tablets, tracking devices, asset tags, and so on. Acommunication link through which UEs can send signals to a RAN is calledan uplink channel (e.g., a reverse traffic channel, a reverse controlchannel, an access channel, etc.). A communication link through whichthe RAN can send signals to UEs is called a downlink or forward linkchannel (e.g., a paging channel, a control channel, a broadcast channel,a forward traffic channel, etc.). As used herein the term trafficchannel (TCH) can refer to either an uplink/reverse or downlink/forwardtraffic channel.

According to various aspects, FIG. 1 illustrates an exemplary wirelesscommunications system 100. The wireless communications system 100 (whichmay also be referred to as a wireless wide area network (WWAN)) mayinclude various base stations 102 and various UEs 104. The base stations102 may include macro cells (high power cellular base stations) and/orsmall cells (low power cellular base stations), wherein the macro cellsmay include Evolved NodeBs (eNBs), where the wireless communicationssystem 100 corresponds to an LTE network, or gNodeBs (gNBs), where thewireless communications system 100 corresponds to a 5G network or acombination of both, and the small cells may include femtocells,picocells, microcells, etc.

The base stations 102 may collectively form a Radio Access Network (RAN)and interface with an Evolved Packet Core (EPC) or Next Generation Core(NGC) through backhaul links. In addition to other functions, the basestations 102 may perform functions that relate to one or more oftransferring user data, radio channel ciphering and deciphering,integrity protection, header compression, mobility control functions(e.g., handover, dual connectivity), inter-cell interferencecoordination, connection setup and release, load balancing, distributionfor non-access stratum (NAS) messages, NAS node selection,synchronization, RAN sharing, multimedia broadcast multicast service(MBMS), subscriber and equipment trace, RAN information management(RIM), paging, positioning, and delivery of warning messages. The basestations 102 may communicate with each other directly or indirectly(e.g., through the EPC/NGC) over backhaul links 134, which may be wiredor wireless.

The base stations 102 may wirelessly communicate with the UEs 104. Eachof the base stations 102 may provide communication coverage for arespective geographic coverage area 110. In an aspect, although notshown in FIG. 1, geographic coverage areas 110 may be subdivided into aplurality of cells (e.g., three), or sectors, each cell corresponding toa single antenna or array of antennas of a base station 102. As usedherein, the term “cell” or “sector” may correspond to one of a pluralityof cells of a base station 102, or to the base station 102 itself,depending on the context.

While neighboring macro cell geographic coverage areas 110 may partiallyoverlap (e.g., in a handover region), some of the geographic coverageareas 110 may be substantially overlapped by a larger geographiccoverage area 110. For example, a small cell base station 102′ may havea geographic coverage area 110′ that substantially overlaps with thegeographic coverage area 110 of one or more macro cell base stations102. A network that includes both small cell and macro cells may beknown as a heterogeneous network. A heterogeneous network may alsoinclude Home eNBs (HeNBs), which may provide service to a restrictedgroup known as a closed subscriber group (CSG). The communication links120 between the base stations 102 and the UEs 104 may include uplink(UL) (also referred to as reverse link) transmissions from a UE 104 to abase station 102 and/or downlink (DL) (also referred to as forward link)transmissions from a base station 102 to a UE 104. The communicationlinks 120 may use MIMO antenna technology, including spatialmultiplexing, beamforming, and/or transmit diversity. The communicationlinks may be through one or more carriers. Allocation of carriers may beasymmetric with respect to DL and UL (e.g., more or less carriers may beallocated for DL than for UL).

The wireless communications system 100 may further include a wirelesslocal area network (WLAN) access point (AP) 150 in communication withWLAN stations (STAs) 152 via communication links 154 in an unlicensedfrequency spectrum (e.g., 5 GHz). When communicating in an unlicensedfrequency spectrum, the WLAN STAs 152 and/or the WLAN AP 150 may performa clear channel assessment (CCA) prior to communicating in order todetermine whether the channel is available.

The small cell base station 102′ may operate in a licensed and/or anunlicensed frequency spectrum. When operating in an unlicensed frequencyspectrum, the small cell base station 102′ may employ LTE or 5Gtechnology and use the same 5 GHz unlicensed frequency spectrum as usedby the WLAN AP 150. The small cell base station 102′, employing LTE/5Gin an unlicensed frequency spectrum, may boost coverage to and/orincrease capacity of the access network. LTE in an unlicensed spectrummay be referred to as LTE-unlicensed (LTE-U), licensed assisted access(LAA), or MulteFire.

The wireless communications system 100 may further include a mmW basestation 180 that may operate in mmW frequencies and/or near mmWfrequencies in communication with a UE 182. Extremely high frequency(EHF) is part of the RF in the electromagnetic spectrum. EHF has a rangeof 30 GHz to 300 GHz and a wavelength between 1 millimeter and 10millimeters. Radio waves in this band may be referred to as a millimeterwave. Near mmW may extend down to a frequency of 3 GHz with a wavelengthof 100 millimeters. The super high frequency (SHF) band extends between3 GHz and 30 GHz, also referred to as centimeter wave. Communicationsusing the mmW/near mmW radio frequency band have high path loss and arelatively short range. The mmW base station 180 may utilize beamforming184 with the UE 182 to compensate for the extremely high path loss andshort range. Further, it will be appreciated that in alternativeconfigurations, one or more base stations 102 may also transmit usingmmW or near mmW and beamforming. Accordingly, it will be appreciatedthat the foregoing illustrations are merely examples and should not beconstrued to limit the various aspects disclosed herein.

The wireless communications system 100 may further include one or moreUEs, such as UE 190, that connects indirectly to one or morecommunication networks via one or more device-to-device (D2D)peer-to-peer (P2P) links. In the embodiment of

FIG. 1, UE 190 has a D2D P2P link 192 with one of the UEs 104 connectedto one of the base stations 102 (e.g., through which UE 190 mayindirectly obtain cellular connectivity) and a D2D P2P link 194 withWLAN STA 152 connected to the WLAN AP 150 (through which UE 190 mayindirectly obtain WLAN-based Internet connectivity). In an example, theD2D P2P links 192-194 may be supported with any well-known D2D radioaccess technology (RAT), such as LTE Direct (LTE-D), WiFi Direct(WiFi-D), Bluetooth, and so on.

According to various aspects, FIG. 2A illustrates an example wirelessnetwork structure 200. For example, an NGC 210 can be viewedfunctionally as control plane functions 214 (e.g., UE registration,authentication, network access, gateway selection, etc.), and user planefunctions 212 (e.g., UE gateway function, access to data networks,Internet protocol (IP) routing, etc.), which operate cooperatively toform the core network. User plane interface (NG-U) 213 and control planeinterface (NG-C) 215 connect the gNB 222 to the NGC 210 and specificallyto the control plane functions 214 and user plane functions 212. In anadditional configuration, an eNB 224 may also be connected to the NGC210 via NG-C 215 to the control plane functions 214 and NG-U 213 to userplane functions 212. Further, eNB 224 may directly communicate with gNB222 via a backhaul connection 223. Accordingly, in some configurations,the New RAN 220 may only have one or more gNBs 222, while otherconfigurations include one or more of both eNBs 224 and gNBs 222. EithergNB 222 or eNB 224 may communicate with UEs 240 (e.g., any of the UEsdepicted in FIG. 1, such as UEs 104, UE 152, UE 182, UE 190, etc.).Another optional aspect may include a location server 230 that may be incommunication with the NGC 210 to provide location assistance for UEs240. The location server 230 can be implemented as a plurality ofstructurally separate servers, or alternately may each correspond to asingle server. The location server 230 can be configured to support oneor more location services for UEs 240 that can connect to the locationserver 230 via the core network, NGC 210, and/or via the Internet (notillustrated). Further, the location server 230 may be integrated into acomponent of the core network, or alternatively may be external to thecore network.

According to various aspects, FIG. 2B illustrates another examplewireless network structure 250. For example, an NGC 260 can be viewedfunctionally as control plane functions, an access and mobilitymanagement function (AMF) 264 and user plane functions, and a sessionmanagement function (SMF) 262, which operate cooperatively to form thecore network. User plane interface 263 and control plane interface 265connect the eNB 224 to the NGC 260 and specifically to AMF 264 and SMF262. In an additional configuration, a gNB 222 may also be connected tothe NGC 260 via control plane interface 265 to AMF 264 and user planeinterface 263 to SMF 262. Further, eNB 224 may directly communicate withgNB 222 via the backhaul connection 223, with or without gNB directconnectivity to the NGC 260. Accordingly, in some configurations, theNew RAN 220 may only have one or more gNBs 222, while otherconfigurations include one or more of both eNBs 224 and gNBs 222. EithergNB 222 or eNB 224 may communicate with UEs 240 (e.g., any of the UEsdepicted in FIG. 1, such as UEs 104, UE 182, UE 190, etc.). Anotheroptional aspect may include a location management function (LMF) 270,which may be in communication with the NGC 260 to provide locationassistance for UEs 240. The LMF 270 can be implemented as a plurality ofseparate servers (e.g., physically separate servers, different softwaremodules on a single server, different software modules spread acrossmultiple physical servers, etc.), or alternately may each correspond toa single server. The LMF 270 can be configured to support one or morelocation services for UEs 240 that can connect to the LMF 270 via thecore network, NGC 260, and/or via the Internet (not illustrated).

According to various aspects, FIG. 3A illustrates an exemplary basestation (BS) 310 (e.g., an eNB, a gNB, a small cell AP, a WLAN AP, etc.)in communication with an exemplary UE 350 (e.g., any of the UEs depictedin FIG. 1, such as UEs 104, UE 152, UE 182, UE 190, etc.) in a wirelessnetwork. In the DL, IP packets from the core network (NGC 210/EPC 260)may be provided to a controller/processor 375. The controller/processor375 implements functionality for a radio resource control (RRC) layer, apacket data convergence protocol (PDCP) layer, a radio link control(RLC) layer, and a medium access control (MAC) layer. Thecontroller/processor 375 provides RRC layer functionality associatedwith broadcasting of system information (e.g., master information block(MIB), system information blocks (SIBs)), RRC connection control (e.g.,RRC connection paging, RRC connection establishment, RRC connectionmodification, and RRC connection release), inter-RAT mobility, andmeasurement configuration for UE measurement reporting; PDCP layerfunctionality associated with header compression/decompression, security(ciphering, deciphering, integrity protection, integrity verification),and handover support functions; RLC layer functionality associated withthe transfer of upper layer packet data units (PDUs), error correctionthrough automatic repeat request (ARQ), concatenation, segmentation, andreassembly of RLC service data units (SDUs), re-segmentation of RLC dataPDUs, and reordering of RLC data PDUs; and MAC layer functionalityassociated with mapping between logical channels and transport channels,scheduling information reporting, error correction, priority handling,and logical channel prioritization.

The transmit (TX) processor 316 and the receive (RX) processor 370implement Layer-1 functionality associated with various signalprocessing functions. Layer-1, which includes a physical (PHY) layer,may include error detection on the transport channels, forward errorcorrection (FEC) coding/decoding of the transport channels,interleaving, rate matching, mapping onto physical channels,modulation/demodulation of physical channels, and MIMO antennaprocessing. The TX processor 316 handles mapping to signalconstellations based on various modulation schemes (e.g., binaryphase-shift keying (BPSK), quadrature phase-shift keying (QPSK),M-phase-shift keying (M-PSK), M-quadrature amplitude modulation(M-QAM)). The coded and modulated symbols may then be split intoparallel streams. Each stream may then be mapped to an orthogonalfrequency-division multiplexing (OFDM) subcarrier, multiplexed with areference signal (e.g., pilot) in the time and/or frequency domain, andthen combined together using an inverse fast Fourier transform (IFFT) toproduce a physical channel carrying a time domain OFDM symbol stream.The OFDM stream is spatially precoded to produce multiple spatialstreams. Channel estimates from a channel estimator 374 may be used todetermine the coding and modulation scheme, as well as for spatialprocessing. The channel estimate may be derived from a reference signaland/or channel condition feedback transmitted by the UE 350. Eachspatial stream may then be provided to one or more different antennas320 via a separate transmitter 318. Each transmitter 318 may modulate anRF carrier with a respective spatial stream for transmission.

At the UE 350, each receiver 354 receives a signal through itsrespective antenna 352. Each receiver 354 recovers information modulatedonto an RF carrier and provides the information to the RX processor 356.The TX processor 368 and the RX processor 356 implement Layer-1functionality associated with various signal processing functions. TheRX processor 356 may perform spatial processing on the information torecover any spatial streams destined for the UE 350. If multiple spatialstreams are destined for the UE 350, they may be combined by the RXprocessor 356 into a single OFDM symbol stream. The RX processor 356then converts the OFDM symbol stream from the time-domain to thefrequency domain using a fast Fourier transform (FFT). The frequencydomain signal comprises a separate OFDM symbol stream for eachsubcarrier of the OFDM signal. The symbols on each subcarrier, and thereference signal, are recovered and demodulated by determining the mostlikely signal constellation points transmitted by the base station 310.These soft decisions may be based on channel estimates computed by thechannel estimator 358. The soft decisions are then decoded andde-interleaved to recover the data and control signals that wereoriginally transmitted by the base station 310 on the physical channel.The data and control signals are then provided to the processing system359, which implements Layer-3 and Layer-2 functionality.

The processing system 359 can be associated with a memory 360 thatstores program codes and data. The memory 360 may be referred to as anon-transitory computer-readable medium. In the UL, the processingsystem 359 provides demultiplexing between transport and logicalchannels, packet reassembly, deciphering, header decompression, andcontrol signal processing to recover IP packets from the core network.The processing system 359 is also responsible for error detection.

Similar to the functionality described in connection with the DLtransmission by the base station 310, the processing system 359 providesRRC layer functionality associated with system information (e.g., MIB,SIBs) acquisition, RRC connections, and measurement reporting; PDCPlayer functionality associated with header compression/decompression,and security (ciphering, deciphering, integrity protection, integrityverification); RLC layer functionality associated with the transfer ofupper layer PDUs, error correction through ARQ, concatenation,segmentation, and reassembly of RLC SDUs, re-segmentation of RLC dataPDUs, and reordering of RLC data PDUs; and MAC layer functionalityassociated with mapping between logical channels and transport channels,multiplexing of MAC SDUs onto transport blocks (TBs), demultiplexing ofMAC SDUs from TBs, scheduling information reporting, error correctionthrough hybrid automatic repeat request (HARQ), priority handling, andlogical channel prioritization.

Channel estimates derived by the channel estimator 358 from a referencesignal or feedback transmitted by the base station 310 may be used bythe TX processor 368 to select the appropriate coding and modulationschemes, and to facilitate spatial processing. The spatial streamsgenerated by the TX processor 368 may be provided to different antenna352 via separate transmitters 354. Each transmitter 354 may modulate anRF carrier with a respective spatial stream for transmission. In anaspect, the transmitters 354 and the receivers 354 may be one or moretransceivers, one or more discrete transmitters, one or more discretereceivers, or any combination thereof.

The UL transmission is processed at the base station 310 in a mannersimilar to that described in connection with the receiver function atthe UE 350. Each receiver 318 receives a signal through its respectiveantenna 320. Each receiver 318 recovers information modulated onto an RFcarrier and provides the information to a RX processor 370. In anaspect, the transmitters 318 and the receivers 318 may be one or moretransceivers, one or more discrete transmitters, one or more discretereceivers, or any combination thereof.

The processing system 375 can be associated with a memory 376 thatstores program codes and data. The memory 376 may be referred to as anon-transitory computer-readable medium. In the UL, the processingsystem 375 provides demultiplexing between transport and logicalchannels, packet reassembly, deciphering, header decompression, controlsignal processing to recover IP packets from the UE 350. IP packets fromthe processing system 375 may be provided to the core network. Theprocessing system 375 is also responsible for error detection.

FIG. 3B illustrates an exemplary server 300B. In an example, the server300B may correspond to one example configuration of the location server230 described above. In FIG. 3B, the server 300B includes a processor301B coupled to volatile memory 302B and a large capacity nonvolatilememory, such as a disk drive 303B. The server 300B may also include afloppy disc drive, compact disc (CD) or DVD disc drive 306B coupled tothe processor 301B. The server 300B may also include network accessports 304B coupled to the processor 301B for establishing dataconnections with a network 307B, such as a local area network coupled toother broadcast system computers and servers or to the Internet.

FIG. 4 illustrates an exemplary wireless communications system 400according to various aspects of the disclosure. In the example of FIG.4, a UE 404, which may correspond to any of the UEs described above withrespect to FIG. 1 (e.g., UEs 104, UE 182, UE 190, etc.), is attemptingto calculate an estimate of its position, or assist another entity(e.g., a base station or core network component, another UE, a locationserver, a third party application, etc.) to calculate an estimate of itsposition. The UE 404 may communicate wirelessly with a plurality of basestations 402 a-d (collectively, base stations 402), which may correspondto any combination of base stations 102 or 180 and/or WLAN AP 150 inFIG. 1, using RF signals and standardized protocols for the modulationof the RF signals and the exchange of information packets. By extractingdifferent types of information from the exchanged RF signals, andutilizing the layout of the wireless communications system 400 (i.e.,the base stations locations, geometry, etc.), the UE 404 may determineits position, or assist in the determination of its position, in apredefined reference coordinate system. In an aspect, the UE 404 mayspecify its position using a two-dimensional coordinate system; however,the aspects disclosed herein are not so limited, and may also beapplicable to determining positions using a three-dimensional coordinatesystem, if the extra dimension is desired. Additionally, while FIG. 4illustrates one UE 404 and four base stations 402, as will beappreciated, there may be more UEs 404 and more or fewer base stations402.

To support position estimates, the base stations 402 may be configuredto broadcast reference RF signals (e.g., Positioning Reference Signals(PRS), Cell-specific Reference Signals (CRS), Channel State InformationReference Signals (CSI-RS), synchronization signals, etc.) to UEs 404 intheir coverage areas to enable a UE 404 to measure reference RF signaltiming differences (e.g., OTDOA or RSTD) between pairs of network nodesand/or to identify the beam that best excite the LOS or shortest radiopath between the UE 404 and the transmitting base stations 402.Identifying the LOS/shortest path beam(s) is of interest not onlybecause these beams can subsequently be used for OTDOA measurementsbetween a pair of base stations 402, but also because identifying thesebeams can directly provide some positioning information based on thebeam direction. Moreover, these beams can subsequently be used for otherposition estimation methods that require precise ToA, such as round-triptime estimation based methods.

As used herein, a “network node” may be a base station 402, a cell of abase station 402, a remote radio head, an antenna of a base station 402,where the locations of the antennas of a base station 402 are distinctfrom the location of the base station 402 itself, or any other networkentity capable of transmitting reference signals. Further, as usedherein, a “node” may refer to either a network node or a UE.

A location server (e.g., location server 230) may send assistance datato the UE 404 that includes an identification of one or more neighborcells of base stations 402 and configuration information for referenceRF signals transmitted by each neighbor cell. Alternatively, theassistance data can originate directly from the base stations 402themselves (e.g., in periodically broadcasted overhead messages, etc.).Alternatively, the UE 404 can detect neighbor cells of base stations 402itself without the use of assistance data. The UE 404 (e.g., based inpart on the assistance data, if provided) can measure and (optionally)report the OTDOA from individual network nodes and/or RSTDs betweenreference RF signals received from pairs of network nodes. Using thesemeasurements and the known locations of the measured network nodes(i.e., the base station(s) 402 or antenna(s) that transmitted thereference RF signals that the UE 404 measured), the UE 404 or thelocation server can determine the distance between the UE 404 and themeasured network nodes and thereby calculate the location of the UE 404.

The term “position estimate” is used herein to refer to an estimate of aposition for a UE 404, which may be geographic (e.g., may comprise alatitude, longitude, and possibly altitude) or civic (e.g., may comprisea street address, building designation, or precise point or area withinor nearby to a building or street address, such as a particular entranceto a building, a particular room or suite in a building, or a landmarksuch as a town square). A position estimate may also be referred to as a“location,” a “position,” a “fix,” a “position fix,” a “location fix,” a“location estimate,” a “fix estimate,” or by some other term. The meansof obtaining a location estimate may be referred to generically as“positioning,” “locating,” or “position fixing.” A particular solutionfor obtaining a position estimate may be referred to as a “positionsolution.” A particular method for obtaining a position estimate as partof a position solution may be referred to as a “position method” or as a“positioning method.”

The term “base station” may refer to a single physical transmissionpoint or to multiple physical transmission points that may or may not beco-located. For example, where the term “base station” refers to asingle physical transmission point, the physical transmission point maybe an antenna of the base station (e.g., base station 402) correspondingto a cell of the base station. Where the term “base station” refers tomultiple co-located physical transmission points, the physicaltransmission points may be an array of antennas (e.g., as in a MIMOsystem or where the base station employs beamforming) of the basestation. Where the term “base station” refers to multiple non-co-locatedphysical transmission points, the physical transmission points may be aDistributed Antenna System (DAS) (a network of spatially separatedantennas connected to a common source via a transport medium) or aRemote Radio Head (RRH) (a remote base station connected to a servingbase station). Alternatively, the non-co-located physical transmissionpoints may be the serving base station receiving the measurement reportfrom the UE (e.g., UE 404) and a neighbor base station whose referenceRF signals the UE is measuring. Thus, FIG. 4 illustrates an aspect inwhich base stations 402 a and 402 b form a DAS/RRH 420. For example, thebase station 402 a may be the serving base station of the UE 404 and thebase station 402 b may be a neighbor base station of the UE 404. Assuch, the base station 402 b may be the RRH of the base station 402 a.The base stations 402 a and 402 b may communicate with each other over awired or wireless link 422.

To accurately determine the position of the UE 404 using the OTDOAsand/or

RSTDs between RF signals received from pairs of network nodes, the UE404 needs to measure the reference RF signals received over the LOS path(or the shortest NLOS path where an LOS path is not available), betweenthe UE 404 and a network node (e.g., base station 402, antenna).However, RF signals travel not only by the LOS/shortest path between thetransmitter and receiver, but also over a number of other paths as theRF signals spread out from the transmitter and reflect off other objectssuch as hills, buildings, water, and the like on their way to thereceiver. Thus, FIG. 4 illustrates a number of LOS paths 410 and anumber of NLOS paths 412 between the base stations 402 and the UE 404.Specifically, FIG. 4 illustrates base station 402 a transmitting over anLOS path 410 a and an NLOS path 412 a, base station 402 b transmittingover an LOS path 410 b and two NLOS paths 412 b, base station 402 ctransmitting over an LOS path 410 c and an NLOS path 412 c, and basestation 402 d transmitting over two NLOS paths 412 d. As illustrated inFIG. 4, each NLOS path 412 reflects off some object 430 (e.g., abuilding). As will be appreciated, each LOS path 410 and NLOS path 412transmitted by a base station 402 may be transmitted by differentantennas of the base station 402 (e.g., as in a MIMO system), or may betransmitted by the same antenna of a base station 402 (therebyillustrating the propagation of an RF signal). Further, as used herein,the term “LOS path” refers to the shortest path between a transmitterand receiver, and may not be an actual LOS path, but rather, theshortest NLOS path.

In an aspect, one or more of base stations 402 may be configured to usebeamforming to transmit RF signals. In that case, some of the availablebeams may focus the transmitted RF signal along the LOS paths 410 (e.g.,the beams produce highest antenna gain along the LOS paths) while otheravailable beams may focus the transmitted RF signal along the NLOS paths412. A beam that has high gain along a certain path and thus focuses theRF signal along that path may still have some RF signal propagatingalong other paths; the strength of that RF signal naturally depends onthe beam gain along those other paths. An “RF signal” comprises anelectromagnetic wave that transports information through the spacebetween the transmitter and the receiver. As used herein, a transmittermay transmit a single “RF signal” or multiple “RF signals” to areceiver. However, as described further below, the receiver may receivemultiple “RF signals” corresponding to each transmitted RF signal due tothe propagation characteristics of RF signals through multipathchannels.

Where a base station 402 uses beamforming to transmit RF signals, thebeams of interest for data communication between the base station 402and the UE 404 will be the beams carrying RF signals that arrive at UE404 with the highest signal strength (as indicated by, e.g., theReceived Signal Received Power (RSRP) or SINR in the presence of adirectional interfering signal), whereas the beams of interest forposition estimation will be the beams carrying RF signals that excitethe shortest path or LOS path (e.g., an LOS path 410). In some frequencybands and for antenna systems typically used, these will be the samebeams. However, in other frequency bands, such as mmW, where typically alarge number of antenna elements can be used to create narrow transmitbeams, they may not be the same beams.

Sidelink communications relate to peer-to-peer communications betweenUEs in accordance with a device-to-device (D2D) protocol (e.g., V2V,V2X, LTE-D, WiFi-Direct, etc.). In some designs, synchronization (e.g.,time and frequency synchronization) is achieved whereby one or more UEsact as a synchronization source (referred to as SyncRef UE). Generally,the peer UEs that belong to a particular sidelink communications networkattempt to maintain a common reference time to facilitate sidelinkcommunications among the peer UEs.

In some designs, sidelink communication links are decoupled fromsidelink synchronization links. For example, two peer UEs participatingin sidelink communication with each other are not required to designateone or the other as a synchronization source for deriving theirrespective time and frequency resources. In some designs, certainsystem-wide resources are designated or reserved for sidelinksynchronization signaling in an SFN-based manner (e.g., in 3GPP Rel. 12,2 resources are reserved for sidelink synchronization signaling at eachsynchronization period). In such an implementation, there is no beammanagement functionality that carries over from sidelink synchronizationto sidelink communication (e.g., because the sidelink synchronizationsignaling is transported via an SFN-based manner).

In some designs, SyncRef UEs can be connected directly to a base station(e.g., gNB) or Global Navigation Satellite System (GNSS), as shown belowwith respect to FIG. 5. In other designs, SyncRef UEs can be indirectlyconnected to the base station or GNSS (e.g., more than one hop away viaone or more peer UEs in the sidelink communications network). In yetother designs, SyncRef UEs can act as independent synchronizationsources without any direct or indirect connection to a base station orGNSS.

FIG. 5 illustrates a sidelink communications network 500 in accordancewith an embodiment of the disclosure. Referring to FIG. 5, the sidelinkcommunications network 500 comprises a GNSS satellite 502 and UEs 504,506, 508 and 510. UE 504 is synchronized with a network clock of theGNSS satellite 502 based on receipt of various GNSS signals. UE 504 isconnected to UE 506 via a sidelink communications link 512, UE 506 isconnected to UE 508 via a sidelink communications link 514, and UE 508is connected to UE 510 via a sidelink communications link 516. While notshown, one or more of UEs 504-510 may also be connected to a terrestrialcommunications network. In FIG. 5, UE 504 corresponds to the SyncRef UE.Also, while not shown, UE 510 may be further connected to yet anotherpeer UE over a sidelink communications link 518, and so on.

As noted above, certain networks reserve 2 resources for sidelinksynchronization signaling. In an example of such a system, the sidelinksynchronization signaling over the sidelink communications links 512-516may be configured as shown in Table 1 (in Table 1, INC corresponds toin-coverage indicator, which indicates if the UE is directlysynchronized either to GNSS or eNB), as follows:

TABLE 1 Link Synchronization Signal Configuration 512 SidelinkSynchronization Signal ID (SLSS ID) = 0 Subframe = Resource 1 INC = True514 SLSS ID = SLSS ID of SyncRef UE (UE 504) + 168 = 0 + 168 Subframe =Resource 2 INC = False 516 SLSS ID = SLSS ID of SyncRef UE (UE 504) = 0Subframe = Resource 1 INC = False 518 SLSS ID = SLSS ID of SyncRef UE(UE 504) = 0 Subframe = Resource 2 INC = False

As shown in Table 1, the Subframe used for the SLSS transmissionalternates at each hop in the sidelink communications network 500between Resources 1 and 2 because there are only two available resourcesfor the SLSS transmissions.

FIG. 6 illustrates a sidelink communications network 600 in accordancewith another embodiment of the disclosure. In FIG. 6, UE 506 and UE 508lose their connection to each other as shown at 602. Hence, UEs 508 and510 are disconnected from the GNSS-synchronized UE 504 which was actingas the SyncRef UE in the sidelink communications network 500 of FIG. 5.UEs 508-510 thereby form a new GNSS-independent sidelink communicationsnetwork. In an example, assume that UE 508 becomes the SyncRef UE forthe new GNSS-independent sidelink communications network. Also, whilenot shown, UE 510 may be further connected to yet another peer UE over asidelink communications link 606, and so on.

In this case, in a system whereby 2 resources are reserved for sidelinksynchronization signaling, the sidelink synchronization signaling oversidelink communications links 604-606 may be configured as shown inTable 2, as follows:

TABLE 2 Link Synchronization Signal Configuration 604 SidelinkSynchronization Signal ID (SLSS ID) = Random (e.g., between 170-355)Subframe = Resource 1 or 2 INC = FALSE 606 SLSS ID = SLSS ID of SyncRefUE (UE 508) + 168 = 0 + 168 Subframe = Resource 1 or 2 (opposite of thatused by UE 508) INC = False

For a UE that derives its synchronization from a SyncRef UE, a referencetiming is the ‘received timing’ of the SyncRef UE's synchronizationsignals (e.g., SFNed) at the receiver (e.g., unsynchronized UE), in amanner that is analogous to downlink timing synchronization with respectto a base station. Sidelink physical channels and signals (forcommunication) may be transmitted based on this reference timing. Insome designs, sidelink communications networks do not support a timingadvance (TA) as in the case of UE-to-gNB uplink. In such sidelinkcommunications networks, the propagation delay along each hop in thesidelink communications network contributes to a timing error betweenthe SyncRef UE and each successive UE at each hop of the sidelinkcommunications network. This timing error depends on the propagationdistance along each hop as well as the number of hops from the originalsynchronization source (e.g., hops from GNSS satellite 502 orterrestrial base station, or the SyncRef UE itself in the case of anunsynchronized network).

FIG. 7 illustrates successively higher timing errors along hops of thesidelink communications network 500 in accordance with an embodiment ofthe disclosure. In particular, timing errors are shown in FIG. 7relative to a particular radio frame denoted as radio frame X. Referringto FIG. 7, UE 504's timing is set to the GNSS timing, UE 506's timing isset to UE 504's timing plus a propagation delay to, UE 508's timing isset to UE 506's timing plus a propagation delay t_(p2), UE 510's timingis set to UE 508's timing plus a propagation delay t_(p3), and so on.Accordingly, the further away a peer UE from the SyncRef UE in terms ofhops, the greater the timing error. Moreover, while FIG. 7 is describedwith respect to the GNSS-synchronized sidelink communications network500 of FIG. 5, the same problem occurs in sidelink communicationsnetworks which lack synchronization with a network clock.

FIG. 8 illustrates an example frame structure 800 that supports sidelinksynchronization signals in accordance with an embodiment of thedisclosure. As shown in FIG. 8, the frame structure 800 includes 14subframes, with subframes 2 and 5 allocated to sidelink secondarysynchronization signals (S-SSS), subframes 3-4 allocated to sidelinkprimary synchronization signals (S-PSS), subframes 6-13 allocated toPSBCH and subframe 14 functioning as a gap. In some designs, thesidelink sync signal block (S-SSB, which comprises S-PSS and S-SSS)periodicity may be 160 ms, although this period may be configurable. Theperiodicity in this context refers to how often the frame structure 800is repeated (e.g., every 160 ms). In some designs, the frame structure800 may be used to support vehicle-based communications, such as NRvehicle-to-everything (V2X) communications. Among other things, theframe structure 800 may be used for sidelink communication-relatedfunctionality, including resource selection, S-SSB ID determination,SyncRef UE selection and/or re-selection, and so on.

Embodiments of the disclosure are directed to mechanisms by which areference timing can be determined in a sidelink communications networkthat takes the propagation delay over one or more hops into account. Insome designs, the reference timing can be calculated in this mannerwhile a SyncRef UE is synchronized with respect to a network clock(e.g., GNSS clock or terrestrial network clock), while in other designs,the reference timing can be calculated in this manner while a SyncRef UEIS unsynchronized with respect to the network clock.

FIG. 9 illustrates an exemplary process 900 of determining a referencetiming according to an aspect of the disclosure. The process 900 of FIG.9 is performed by a UE, which may correspond to any of the above-notedUEs (e.g., UE 240, 350, 504, 506, 508, 510, etc.). At 902, the UE (e.g.,controller/processor 359, antenna(s) 352, receiver(s) 354, RX processor356, transmitter(s) 354, and/or TX processor 368) establishes, with apeer sidelink UE, at least one sidelink communications link that eachcomprises one or more hops.

At 904, the UE (e.g., controller/processor 359, antenna(s) 352,receiver(s) 354,

RX processor 356, transmitter(s) 354, and/or TX processor 368)optionally determines while the UE is synchronized with respect to anetwork clock (e.g., a GNSS clock, a terrestrial network clock, etc.), apropagation delay parameter between the UE and a peer sidelink UE. Insome designs, the determination performed by the UE at 904 is optionalbecause the propagation delay parameter can instead be obtained at leastin part via crowdsourcing from one or more other UEs. For example, thepropagation delay parameter can be determined by another UE that is (orwas previously) located in proximity to the UE's current location, andthen forwarded to the UE (e.g., either directly or indirectly). Inanother example, the propagation delay parameter may be averaged frompropagation delay parameters determined by a plurality of such UEs(e.g., a weighted average, whereby more recently determined propagationdelay parameters or propagation delay parameters determined in closerproximity to the UE's current location are prioritized more highly thanother propagation delay parameters, etc.). In a further example, thepropagation delay parameter may be determined by the UE at 904 and thenmay itself be averaged or weighted based on one or more crowdsourcedpropagation delay parameter(s). In some designs, irrespective of whetherthe UE or some other UE or combination of UEs determines the propagationdelay parameter, each UE whose measurements contribute to thepropagation delay parameter in some manner is synchronized with respectto the network clock when such measurements are made. In one embodiment,the crowdsource information may be collated and averaged at a server, anetwork edge and/or road-side unit (RSU), and thereafter provided to theUE.

With respect to 904, the network clock synchronization can either bedirect or indirect. For example, in context with FIG. 5, UE 504 isdirectly synchronized with the GNSS clock of GNSS satellite 502, whereasUEs 506-510 are indirectly synchronized with the GNSS clock of GNSSsatellite 502 via their respective sidelink hops to UE 504. In contextwith FIG. 6, UE 504 is directly synchronized with the GNSS clock of GNSSsatellite 502, UE 506 is indirectly synchronized with the GNSS clock ofGNSS satellite 502 via its respective sidelink hop to UE 504, and UEs508-508 are unsynchronized with the GNSS clock of GNSS satellite 502.

At 906, the UE (e.g., controller/processor 359) estimates, while the UEis synchronized with respect to the network clock, a propagation delaybetween the UE and the peer sidelink UE based in part upon thepropagation delay parameter. As will be described below in more detail,the propagation delay parameter can correspond to a calculatedpropagation delay between the UE and the peer sidelink UE. In this case,the estimating of 906 simply reuses the propagation delay that wasdetermined at 904. Alternatively, the propagation delay parameter cancorrespond to a relationship between various metrics by which thepropagation delay (or propagation time) can be estimated. In this case,the estimating of 906 may involve determining these metrics and thenestimating the propagation delay (or propagation time) as a function ofthe determined relationship. The relationship may be determined locallyat the UE, or may be crowdsourced from one or more other UEs, or acombination thereof. These aspects will be explained in more detailbelow.

At 908, the UE (e.g., controller/processor 359) determines, while the UEis synchronized with respect to the network clock, a reference timingbased on the estimated propagation delay. In an example, the referencetiming determination 908 may be performed with respect to Radio Frame Xas in FIG. 7, except a respective UE at each hop compensates for thepropagation delay on that hop (t_(p1), t_(p2), or t_(p3)) such that theRadio Frame X does not drift as the hop count increases as shown in FIG.7.

FIG. 10 illustrates an exemplary process 1000 of determining a referencetiming according to another aspect of the disclosure. The process 1000of FIG. 10 is performed by a UE, which may correspond to any of theabove-noted UEs (e.g., UE 240, 350, 504, 506, 508, 510, etc.). At 1002,the UE (e.g., controller/processor 359, antenna(s) 352, receiver(s) 354,RX processor 356, transmitter(s) 354, and/or TX processor 368)establishes, with a peer sidelink UE, at least one sidelinkcommunications link that each comprises one or more hops.

At 1004, the UE (e.g., controller/processor 359, antenna(s) 352,receiver(s) 354,

RX processor 356, transmitter(s) 354, and/or TX processor 368)determines while the UE is synchronized with respect to a network clock(e.g., a GNSS clock, a terrestrial network clock, etc.), a propagationdelay parameter between the UE and a peer sidelink UE. In an example,1002-1004 may correspond to 902-904 of FIG. 9. In some designs, thedetermination performed by the UE at 1004 is optional because thepropagation delay parameter can instead be obtained at least in part viacrowdsourcing from one or more other UEs. For example, the propagationdelay parameter can be determined by another UE that is (or waspreviously) located in proximity to the UE's current location, and thenforwarded to the UE (e.g., either directly or indirectly). In anotherexample, the propagation delay parameter may be averaged frompropagation delay parameters determined by a plurality of such UEs(e.g., a weighted average, whereby more recently determined propagationdelay parameters or propagation delay parameters determined in closerproximity to the UE's current location are prioritized more highly thanother propagation delay parameters, etc.). In a further example, thepropagation delay parameter may be determined by the UE at 1004 and thenmay itself be averaged or weighted based on one or more crowdsourcedpropagation delay parameter(s). In some designs, irrespective of whetherthe UE or some other UE or combination of UEs determines the propagationdelay parameter, each UE whose measurements contribute to thepropagation delay parameter in some manner is synchronized with respectto the network clock when such measurements are made. In one embodiment,the crowdsource information may be collated and averaged at a server, anetwork edge and/or RSU and thereafter provided to the UE.

At some point after 1004, assume that the UE becomes unsynchronized withrespect to the network clock (e.g., as shown in FIG. 6 with respect toUEs 508 and 510). With respect to 1006, the UE (e.g.,controller/processor 359) estimates, while the UE is unsynchronized withrespect to the network clock, a propagation delay (or propagation time)between the UE and the peer sidelink UE based in part upon thepropagation delay parameter that was determined while the UE wassynchronized with respect to the network clock. Hence, even though thepropagation delay parameter may be somewhat out-of-date, the propagationdelay parameter is leveraged for some period of time. As will beappreciated described in more detail below, if the propagation delayparameter comprises a relationship between metrics, up-to-date values ofthose metrics can be ascertained and then applied to the predeterminedrelationship to derive an estimate of the propagation delay.Alternatively, if the propagation delay parameter corresponds to anearlier calculated propagation delay, that earlier calculatedpropagation delay (from when the UE 1005 was synchronized) can simply beused as the estimated propagation delay. The relationship may bedetermined locally at the UE, or may be crowdsourced from one or moreother UEs, or a combination thereof.

At 1008, the UE (e.g., controller/processor 359) determines, while theUE is unsynchronized with respect to the network clock, a referencetiming based on the estimated propagation delay. In an example, thereference timing determination 1008 may be performed with respect toRadio Frame X as in FIG. 7, except a respective UE at each hopcompensates for the propagation delay on that hop (t_(p1), t_(p2), ort_(p3)) such that the Radio Frame X does not drift as the hop countincreases as shown in FIG. 7.

FIG. 11 illustrates an example implementation of the processes 900-1000in accordance with an embodiment of the disclosure. The exemplaryprocess of FIG. 11 is described with respect to the embodiment wherebyUE 1 and/or UE 2 determine their own respective propagation delayparameter(s) (e.g., propagation time to RSRP relationship). However, inother embodiments, the determination of the propagation delayparameter(s) may be at least partially crowdsourced.

At 1102, a network (e.g., GNSS satellite 502, base station 310, etc.)transmits timing signal(s) that are based on a network clock, and UE 2receives the timing signals at 1104. At 1106, UE 2 synchronizes with thenetwork clock based on the received timing signal(s). At 1108, UEs 1 and2 establish a sidelink communications link (e.g., as in 902 of FIG. 9 or1002 of FIG. 10). At 1110, UE 1 and/or UE 2 determine a propagationdelay parameter between UEs 1 and 2 (e.g., as in 904 of FIGS. 9 and 1004of FIG. 10). At this point, UE 2 (and possibly UE 1 as well) issynchronized with respect to the network clock. At 1112, UE 1 and/or UE2 estimate a propagation delay based on the propagation delay parameter(e.g., as in 906 of FIG. 9). At 1114, UE 1 and/or UE 2 determine areference timing based on the estimated propagation delay from 1112(e.g., as in 908 of FIG. 9).

Referring to FIG. 11, at 1116, the network transmits timing signal(s)that are based on the network clock, but the timing signal(s) are lostat 1118 (i.e., not successfully received by UE 2). At 1120, both UEs 1and 2 are unsynchronized with the network clock due to the loss of thetiming signal(s). At 1122, UE 1 and/or UE 2 estimate the propagationdelay based on the propagation delay parameter determined at 1110 (e.g.,as in 1006 of FIG. 10). At 1124, UE 1 and/or UE 2 determine a referencetiming based on the estimated propagation delay from 1122 (e.g., as in908 of FIG. 9).

In some designs, the propagation delay parameter described above withrespect to FIGS. 8-11 may comprise a relationship between a propagationtime and Reference Signal Received Power (RSRP). The RSRP is typicallyenvironmentally dependent so the above-noted relationship cannot beapplied globally between peer UEs. However, a propagation-to-RSRPrelationship may generally be reliable for some threshold period of timedepending on the stability of the environment and UE mobility. In anexample, the relationship can be computed while a SyncRef UE issynchronized with the network clock with a known location. At this time,RSRP is measured and parameters that characterize thepropagation-to-RSRP relationship are calculated. One examplepropagation-to-RSRP relationship (or function f( ) ) is as follows:

RSRP=const*(distance)^(alpha);

f(RSRP)=(RSRP/a)^(b),

whereby the parameters ‘a’ and ‘b’ can later be reused to estimate thepropagation delay while the respective UEs are unsynchronized, const isa constant value based on the well known free-space path loss (FSPL)formula which derives from the Friis transmission formula, and alpha isa path-loss value (e.g., in free space alpha =2, in an environment withreflections such as multi-path alpha may range between 2.5 to 4, etc.).

In an example, sidelink synchronization signals can be used to derivethe propagation-to-RSRP relationship. However, this is not strictlynecessary, and other sidelink signals can also be used. For example, aDMRS over a sidelink communication channel can be used to derive thepropagation-to-RSRP relationship (e.g., so long as the sidelinkcommunication includes the synchronization status of the transmittingUE). Hence, any reference signal (e.g., DMRS, CSI-RS, etc.) orsynchronization signals (SSB) from a transmitting UE that it itselfsynchronized (so that its Tx timing is accurate up to allowed limits)can be used to facilitate the determination of the propagation-to-RSRPrelationship. Later, when the respective UEs lose their network clocksynchronization, the RSRP (e.g., of sidelink synchronization signals)can be measured while unsynchronized with the predeterminedpropagation-to-RSRP relationship being used to estimate the propagationtime (while unsynchronized).

More specifically, in some designs, when unsynchronized, the referencetiming can be derived using a sidelink synchronization signaltransmitted by a peer UE based on a function of the received timing ofthe synchronization signal from the peer UE, the RSRP of the sidelinksynchronization signal, and an estimate of the propagation time usingthe predetermined propagation-to-RSRP relationship (f( ) ).

In one example, the timing reference determination at 908 or 1008comprises:

-   Estimating a first reference time (t1) as the time of reception of a    synchronization signal transmitted by a SyncRef UE,-   Estimating the RSRP of the synchronization signal,-   Estimating a second reference time (t2) as the t2=t1−f⁻¹(RSRP),    Using the second reference time (t2) as the timing reference for    transmission of sidelink physical signals and waveforms to one or    more peer UE(s).

In another example, the determination of the propagation-to-RSRPrelationship (f() ) may comprise determining a propagation time (t_(pd))estimate based on a location of UE and a location of the peer UE (e.g.,while synchronized). In one case, where the location of the peer UE isknown at the UE using location information included as part of asidelink transmission from the peer UE. Such a transmission may occur ata time prior to the transmission of the synchronization signal, or atthe same time (e.g., as part of a sidelink data channel). A RSRPestimate of the received sidelink signal is then determined. One or moreparameters of a propagation-to-RSRP relationship (f( ) ) are thendetermined which equate (or map) the measured RSRP with the propagationtime (t_(pd)) estimate. In one example, where RSRP=f(t_(pd)), f( ) hasthe parametric form f(t_(pd))=a*(t_(pd))^(b).

In another example, when synchronized, a quality of the synchronizationbetween the peer UEs may be ascertained. In one example, the quality canbe indicated (e.g., in sidelink synchronization signals) as a levelbetween 0 and 1. For example, 1 can be used to designate high qualityGNSS synchronization, where an expected timing error is small (e.g.,less than 3 Ts, etc.). In another example, 0.5 can be used to designatelower quality GNSS synchronization, where an expected timing error islarger (e.g., less than 12 Ts, etc.). In some designs, thesynchronization quality can be used as a weighting coefficient in thepropagation-to-RSRP relationship (f( ) ).

In some designs, the propagation-to-RSRP relationship (f( ) ) may dependon the receive beam (e.g., spatial configuration of the receive beam).In this case, when synchronized, the propagation-to-RSRP relationship(f( ) ) is determined specific to a particular receive beam of the UE.Then, when unsynchronized, the propagation-to-RSRP relationship (f( ) )is likewise used to determine the reference timing for that specificreceive beam. For example, parameters (a,b) in the propagation-to-RSRPrelationship (f( ) ) may be different based on whether a particularreceive beam is LOS or NLOS. Specific recognition of whether a beam isLOS or NLOS is possible but not expressly required.

In some designs, the propagation delay parameter need not include thepropagation-to-RSRP relationship (f( ) ) as described above. Forexample, when synchronized, a UE can estimate an arrival time of asidelink synchronization signal from a SyncRef UE to determine a one-waypropagation delay between the two UEs. It will be appreciated that therecan sometimes be multiple SyncRef UEs, in which case the propagationdelay can be estimated with respect to multiple SyncRef UEs. Then, whenunsynchronized, the UE continues to receive the sidelink synchronizationsignals from the SyncRef UE(s) (e.g., some of which may stoptransmitting the sidelink synchronization signal if out of coverage) anduse the previously recorded propagation delay(s) to estimate the currentpropagation delay(s). So, while the above-noted propagation-to-RSRPrelationship (f( ) ) relies upon a combination of old data and new data(e.g., the current RSRP), in this embodiment the ‘old’ propagation delayis simply re-used.

In further designs, the various operations described above with respectto FIGS. 9-10 may be implemented via various “means”, such as particularhardware components of the associated UEs 905 and 1005. For example,means for performing the establishing aspects of 502, 504, 802 and 804may correspond to any combination of transceiver-related circuitry onthe respective UEs, such as antenna(s) 352, receiver(s) 354, RXprocessor 356, transmitter(s) 354, Tx processor 368, etc. of UE 350 ofFIG. 3A. In a further example, means for performing the determining andestimating aspects of 904-908 and 1004-1008 may corresponding to anycombination of processor-related circuitry on the respective UEs, suchas controller/processor 359 of UE 350 of FIG. 3A.

While some of the embodiments are described above with respect to EN-DCmode, the various embodiments of the disclosure are also applicable withrespect to other types of dual connectivity modes, such as such as NR-NRNR-LTE, etc. Moreover, while some of the embodiments are described withrespect to specific numerologies (e.g., 15 kHz SCS), other embodimentsmay be directed to implementations whereby different numerologies areused (e.g., 30 kHz SCS, 60 kHz SCS, 120 kHz SCS, 240 kHz SCS, 480 kHzSCS, etc.).

Those skilled in the art will appreciate that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

Further, those skilled in the art will appreciate that the variousillustrative logical blocks, modules, circuits, and algorithm stepsdescribed in connection with the aspects disclosed herein may beimplemented as electronic hardware, computer software, or combinationsof both. To clearly illustrate this interchangeability of hardware andsoftware, various illustrative components, blocks, modules, circuits,and steps have been described above generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application,but such implementation decisions should not be interpreted to departfrom the scope of the various aspects described herein.

The various illustrative logical blocks, modules, and circuits describedin connection with the aspects disclosed herein may be implemented orperformed with a general purpose processor, a digital signal processor(DSP), an application specific integrated circuit (ASIC), a fieldprogrammable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.A general purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices (e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or other suchconfigurations).

The methods, sequences, and/or algorithms described in connection withthe aspects disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in RAM, flash memory, ROM, EPROM, EEPROM,registers, hard disk, a removable disk, a CD-ROM, or any other form ofnon-transitory computer-readable medium known in the art. An exemplarynon-transitory computer-readable medium may be coupled to the processorsuch that the processor can read information from, and write informationto, the non-transitory computer-readable medium. In the alternative, thenon-transitory computer-readable medium may be integral to theprocessor. The processor and the non-transitory computer-readable mediummay reside in an ASIC. The ASIC may reside in a user device (e.g., a UE)or a base station. In the alternative, the processor and thenon-transitory computer-readable medium may be discrete components in auser device or base station.

In one or more exemplary aspects, the functions described herein may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on a non-transitorycomputer-readable medium. Computer-readable media may include storagemedia and/or communication media including any non-transitory mediumthat may facilitate transferring a computer program from one place toanother. A storage media may be any available media that can be accessedby a computer. By way of example, and not limitation, suchcomputer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or otheroptical disk storage, magnetic disk storage or other magnetic storagedevices, or any other medium that can be used to carry or store desiredprogram code in the form of instructions or data structures and that canbe accessed by a computer. Also, any connection is properly termed acomputer-readable medium. For example, if the software is transmittedfrom a website, server, or other remote source using a coaxial cable,fiber optic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave, then the coaxial cable, fiber opticcable, twisted pair, DSL, or wireless technologies such as infrared,radio, and microwave are included in the definition of a medium. Theterm disk and disc, which may be used interchangeably herein, includesCD, laser disc, optical disc, DVD, floppy disk, and Blu-ray discs, whichusually reproduce data magnetically and/or optically with lasers.Combinations of the above should also be included within the scope ofcomputer-readable media.

While the foregoing disclosure shows illustrative aspects, those skilledin the art will appreciate that various changes and modifications couldbe made herein without departing from the scope of the disclosure asdefined by the appended claims. Furthermore, in accordance with thevarious illustrative aspects described herein, those skilled in the artwill appreciate that the functions, steps, and/or actions in any methodsdescribed above and/or recited in any method claims appended hereto neednot be performed in any particular order. Further still, to the extentthat any elements are described above or recited in the appended claimsin a singular form, those skilled in the art will appreciate thatsingular form(s) contemplate the plural as well unless limitation to thesingular form(s) is explicitly stated.

What is claimed is:
 1. A method of operating a user equipment (UE),comprising: establishing, with a peer sidelink UE, at least one sidelinkcommunications link that each comprises one or more hops; estimating,while the UE is unsynchronized with respect to a network clock, apropagation delay between the UE and the peer sidelink UE based in partupon a relationship between a propagation time, between the UE and thepeer sidelink UE, and Reference Signal Received Power (RSRP); anddetermining, while the UE is unsynchronized with respect to the networkclock, a reference timing based on the estimated propagation delay. 2.The method of claim 1, wherein the network clock is for a GlobalNavigation Satellite System (GNSS) or a terrestrial network.
 3. Themethod of claim 1, wherein the relationship is determined locally at theUE, or wherein the relationship is crowdsourced from one or more otherUEs, or a combination thereof.
 4. The method of claim 1, wherein theestimating estimates the propagation delay as a function of a currentRSRP based on the relationship.
 5. The method of claim 4, wherein thecurrent RSRP is measured with respect to a sidelink synchronizationsignal.
 6. The method of claim 1, wherein the relationship between theRSRP and the propagation time is based at least in part on measurementof a reference signal or a sidelink synchronization signal that occurswhile the UE or another UE is synchronized with respect to the networkclock.
 7. A user equipment (UE), comprising: means for establishing,with a peer sidelink UE, at least one sidelink communications link thateach comprises one or more hops; means for estimating, while the UE isunsynchronized with respect to a network clock, a propagation delaybetween the UE and the peer sidelink UE based in part upon arelationship between a propagation time, between the UE and the peersidelink UE, and Reference Signal Received Power (RSRP); and means fordetermining, while the UE is unsynchronized with respect to the networkclock, a reference timing based on the estimated propagation delay. 8.The UE of claim 7, wherein the network clock is for a Global NavigationSatellite System (GNSS) or a terrestrial network.
 9. The UE of claim 7,wherein the relationship is determined locally at the UE, or wherein therelationship is crowdsourced from one or more other UEs, or acombination thereof.
 10. The UE of claim 7, wherein the means forestimating estimates the propagation delay as a function of a currentRSRP based on the relationship.
 11. The UE of claim 10, wherein thecurrent RSRP is measured with respect to a sidelink synchronizationsignal.
 12. The UE of claim 7, wherein the relationship between the RSRPand the propagation time is based at least in part on measurement of areference signal or a sidelink synchronization signal that occurs whilethe UE or another UE is synchronized with respect to the network clock.13. A user equipment (UE), comprising: a memory; at least onetransceiver; and at least one processor coupled to the memory and the atleast the transceiver, the at least one processor configured to:establish, with a peer sidelink UE, at least one sidelink communicationslink that each comprises one or more hops; estimate, while the UE isunsynchronized with respect to a network clock, a propagation delaybetween the UE and the peer sidelink UE based in part upon arelationship between a propagation time, between the UE and the peersidelink UE, and Reference Signal Received Power (RSRP); and determine,while the UE is unsynchronized with respect to the network clock, areference timing based on the estimated propagation delay.
 14. The UE ofclaim 13, wherein the network clock is for a Global Navigation SatelliteSystem (GNSS) or a terrestrial network.
 15. The UE of claim 13, whereinthe relationship is determined locally at the UE, or wherein therelationship is crowdsourced from one or more other UEs, or acombination thereof.
 16. The UE of claim 13, wherein the at least oneprocessor is configured to estimate the propagation delay as a functionof a current RSRP based on the relationship.
 17. The UE of claim 16,wherein the current RSRP is measured with respect to a sidelinksynchronization signal.
 18. The UE of claim 13, wherein the relationshipbetween the RSRP and the propagation time is based at least in part onmeasurement of a reference signal or a sidelink synchronization signalthat occurs while the UE or another UE is synchronized with respect tothe network clock.
 19. A non-transitory computer-readable mediumcontaining instructions stored thereon, which, when executed by a userequipment (UE), cause the UE to perform actions, the instructionscomprising: at least one instruction configured to cause the UE toestablish, with a peer sidelink UE, at least one sidelink communicationslink that each comprises one or more hops; at least one instructionconfigured to cause the UE to estimate, while the UE is unsynchronizedwith respect to a network clock, a propagation delay between the UE andthe peer sidelink UE based in part upon a relationship between apropagation time, between the UE and the peer sidelink UE, and ReferenceSignal Received Power (RSRP); and at least one instruction configured tocause the UE to determine, while the UE is unsynchronized with respectto the network clock, a reference timing based on the estimatedpropagation delay.
 20. The non-transitory computer-readable medium ofclaim 19, wherein the network clock is for a Global Navigation SatelliteSystem (GNSS) or a terrestrial network.
 21. The non-transitorycomputer-readable medium of claim 19, wherein the relationship isdetermined locally at the UE, or wherein the relationship iscrowdsourced from one or more other UEs, or a combination thereof. 22.The non-transitory computer-readable medium of claim 19, wherein the atleast one instruction configured to cause the UE to estimate causes theUE to estimate the propagation delay as a function of a current RSRPbased on the relationship.
 23. The non-transitory computer-readablemedium of claim 22, wherein the current RSRP is measured with respect toa sidelink synchronization signal.
 24. The non-transitorycomputer-readable medium of claim 19, wherein the relationship betweenthe RSRP and the propagation time is based at least in part onmeasurement of a reference signal or a sidelink synchronization signalthat occurs while the UE or another UE is synchronized with respect tothe network clock.